WASHINGTON — For the third time in as many years, the U.S. Department of Energy (DoE) and NASA are asking Congress to fund a multiyear effort to re-establish a domestic capability to produce plutonium-238, a vital power source for certain deep space missions.

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Brian Berger is editor in chief of SpaceNews.com and the SpaceNews magazine. He joined SpaceNews.com in 1998, spending his first decade with the publication covering NASA. He was named senior staff writer in 2004, a position he held until being named...